Duty cycle control for motor/LED in Embedded C - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of this PWM duty cycle calculation?

Given the following code snippet that calculates the PWM duty cycle for a motor, what is the value of duty_cycle after execution?

Embedded C

int max_pwm = 255;
int speed_percent = 60;
int duty_cycle = (speed_percent * max_pwm) / 100;

A 255

B 153

C 60

D 100

💡 Hint

Remember that duty cycle is a fraction of max PWM value based on percentage.

The duty cycle is calculated by multiplying the speed percentage (60) by the max PWM value (255) and dividing by 100, which results in 153.

❓ Predict Output

intermediate

What is the LED brightness value after this PWM update?

Consider this code controlling LED brightness with PWM. What is the value of led_brightness after running this code?

Embedded C

int max_pwm = 1023;
int brightness_percent = 25;
int led_brightness = (brightness_percent * max_pwm) / 100;

A 256

B 255

C 1023

D 25

💡 Hint

Calculate the percentage of max PWM value.

25% of 1023 is 255.75, which truncates to 255 in integer arithmetic, but the actual integer division results in 255.75 truncated to 255. However, the calculation (25 * 1023) / 100 = 255.75 truncated to 255, so option A is correct.

❓ Predict Output

advanced

What error does this PWM duty cycle code produce?

What error will occur when compiling or running this code snippet?

Embedded C

int max_pwm = 255;
float speed_percent = 75.5;
int duty_cycle = (speed_percent * max_pwm) / 100;

A TypeError: cannot assign float to int without cast

B No error, duty_cycle = 192

C SyntaxError: invalid expression

D Warning: implicit conversion from float to int, duty_cycle = 192

💡 Hint

Check how float to int conversion works in C.

In C, assigning a float expression to an int variable causes implicit conversion with truncation, usually with a warning but no error. The value is 192 after truncation.

🧠 Conceptual

advanced

Which statement best describes PWM duty cycle effect on motor speed?

Select the statement that correctly explains how PWM duty cycle affects motor speed.

A Higher duty cycle means the motor receives power for a longer time, increasing speed.

B PWM frequency controls speed, duty cycle controls torque.

C Duty cycle does not affect motor speed, only voltage does.

D Lower duty cycle increases motor speed by reducing power interruptions.

💡 Hint

Think about how power delivery time affects motor rotation.

PWM duty cycle controls how long the motor receives power in each cycle. A higher duty cycle means more power time, increasing speed.

❓ Predict Output

expert

What is the final value of duty_cycle after this code runs?

Analyze this embedded C code that adjusts PWM duty cycle dynamically. What is the final value of duty_cycle?

Embedded C

int max_pwm = 255;
int duty_cycle = 0;
for (int i = 0; i < 5; i++) {
    duty_cycle += (i * max_pwm) / 10;
}
if (duty_cycle > max_pwm) {
    duty_cycle = max_pwm;
}

A 0

B 127

C 254

D 255

💡 Hint

Calculate the sum inside the loop and check the cap condition.

The loop adds (i * 255) / 10 for i=0 to 4: 0 + 25 + 51 + 76 + 102 = 254. Since 254 <= 255, the cap condition is not met, so duty_cycle = 254 (A).